Glasses and glass-ceramics with metal alloy surfaces

ABSTRACT

The subject invention is drawn to the production of composite glass articles having base compositions within the Li 2  O and/or Na 2  O-FeO-CoO and/or NiO-Al 2  O 3  SiO 2  system and glass-ceramic articles having base compositions within the Li 2  O-FeO-CoO and/or NiO-Al 2  O 3  -SiO 2  system wherein the articles have a thin, integral, tightly-bonded surface layer containing crystals of nickel-iron, cobalt-iron, or cobalt-nickel-iron alloy.

BACKGROUND OF THE INVENTION

U.S. Pat. No. 4,083,709 discloses the production of glass-ceramicarticles having integral surface layers exhibiting ferrimagneticproperties wherein the interior portion thereof consists essentially ofbeta-spodumene solid solution crystals dispersed within a glassy matrixand the surface layer consists essentially of CoFe₂ O₄ and/or NiFe₂ O₄crystallites dispersed within a glassy matrix. Such articles areprepared by melting a predetermined batch in the Li₂ O-Al₂ O₃ -Fe₂ O₃-NiO and/or CoO-SiO₂ -TiO₂ composition field, simultaneously cooling themelt and shaping a glass article therefrom, and then exposing the glassarticle in an oxidizing atmosphere to temperatures between 900°-1300° C.to effect the growth in situ of the desired beta-spodumene solidsolution crystals in the body portion of the article and of CoFe₂ O₄and/or NiFe₂ O₄ crystallites in the integral surface layer.

U.S. Application Ser. No. 946,809, filed concurrently herewith by thepresent applicant, describes the production of composite glass articleshaving a glassy interior portion and a thin, integral surface layercontaining CoFe₂ O₄, NiFe₂ O₄, and (Co,Ni)Fe₂ O₄ crystallitesdemonstrating ferrimagnetic properties. Such articles are prepared bymelting a proper batch selected from the Li₂ O and/or Na₂ O-FeO-CoOand/or NiO-Al₂ O₃ -SiO₂ composition field, simultaneously cooling themelt and shaping a glass article therefrom, and subsequently exposingthe glass article in an oxidizing environment to temperatures between725°-875° C. to effect the desired in situ growth of CoFe₂ O₄, NiFe₂ O₄,and (Co,Ni)Fe₂ O₄ crystallites in the surface of the article.

U.S. Pat. No. 3,962,514 discusses the manufacture of glass-ceramicarticles containing beta-quartz and/or beta-spodumene solid solution asthe predominant crystal phase in the interior portion thereof and whichhave an integral surface layer containing transition metal spinel-typecrystals. As illustrative of such spinel-type crystals, the patentreported, among others, NiAl₂ O₄, CoAl₂ O₄, CoFe₂ O₄, and NiFe₂ O₄.

The base glasses for such articles are stated to reside in the Al₂ O₃-SiO₂ and Al₂ O₃ -B₂ O₃ systems with Li₂ O or MnO₂ as the primarymodifying metal oxide. The glasses are nucleated with TiO₂ and/or ZrO₂.The transition metal oxides are present in amounts between 0.1-10% toform the spinel-type crystals.

The precursor glass articles are crystallized in situ to glass-ceramicarticles by heating between 800°-1200° C. Thereafter, the integralsurface layer containing spinel-type crystals is developed in situ byfiring the glass-ceramic article in a reducing environment at 500°-1000°C. In the preferred practice, the glass-ceramic article is treated in anacid solution prior to firing in a reducing atmosphere.

The preparation of glass-ceramic articles had its genesis in U.S. Pat.No. 2,920,971 and reference is hereby made to that patent for furtherinformation regarding the physical properties and internal structures ofglass-ceramic articles, as well as the mechanism and temperatureparameters involved in the nucleating and crystallization of sucharticles. The glass-ceramic articles of the instant invention, asdescribed hereinafter, are prepared utiizing the general methoddescribed and have a microstructure similar to that described in thatdisclosure.

OBJECTIVE OF THE INVENTION

The principal objective of the present invention is to provide compositeglass and glass-ceramic articles having thin, integral surface layersdemonstrating metallic, ferromagnetic properties resulting from thepresence of cobalt-iron, nickel-iron, or cobalt-nickel-iron alloycrystals therein.

Another objective of the invention is to provide a method for makingsuch composite glass and glass-ceramic articles.

SUMMARY OF THE INVENTION

I have found that those objectives with respect to glass articles can beaccomplished employing base compositions within the Li₂ O and/or Na₂O-FeO-CoO and/or NiO-Al₂ O₃ -SiO₂ field and, where a glass-ceramicarticle is desired, by including a nucleating agent such as TiO₂ with,optionally, ZrO₂ in a base composition within the Li₂ O-FeO-CoO and/orNiO-Al₂ O₃ -SiO₂ field.

Thus, the inventive composite glass articles consist of an interiorglassy portion and a very thin, highly crystalline, integral surfacelayer consisting of very fine-grained crystals of cobalt-iron,nickel-iron, or the ternary cobalt-nickel-iron alloy dispersed within aglassy matrix, the surface layer not exceeding several thousandangstroms in thickness and the crystals having diameters less than onemicron and, normally, less than 0.5 micron. The glass articles have anoverall composition consisting essentially, in weight percent on theoxide basis, of about 1-17% R₂ O, wherein R₂ O consists of Li₂ O and/orNa₂ O in the following indicated proportions when either is presentalone of 2.5-5.5% Li₂ O and 1-16% Na₂ O, 1-3.75% FeO, 0.75-5% RO,wherein RO consists of NiO and/or CoO in the following indicatedproportions of 0.75-3% NiO and 0.75-4% CoO, 20-32% Al₂ O₃, and 50-72%SiO₂. Glass formers such as P₂ O₅ and B₂ O₃ may be included in amountsof up to 10% P₂ O₅ and up to 3% B₂ O₃. Fluoride may also be added to thebase composition in quantities up to about 2% as a fluxing agent. Thepresence of B₂ O₃ and/or F also appears to be beneficial in promotingthe growth of ferrite crystals, viz., CoFe₂ O₄, NiFe₂ O₄, and composite(Co,Ni)Fe₂ O₄. As₂ O₃ will customarily be included to perform itsconventional function as a fining agent. The preferred compositions willconsist essentially solely of Li₂ O and/or Na₂ O, FeO, CoO and/or NiO,Al₂ O₃, and SiO₂ with, optionally, P₂ O₅, B₂ O₃, and/or F. The commonnucleating agents such as TiO₂, SnO₂, and ZrO₂ will desirably be absentfrom the compositions in order to enhance the low temperaturedevelopment of ferrite crystallites in the surface and to inhibit thegrowth in situ of internal crystallization.

The inventive composite glass-ceramic articles consist of a highlycrystalline, interior portion consisting essentially of beta-spodumenesolid solution crystals dispersed within a glassy matrix and a verythin, highly crystalline, integral surface layer consisting of veryfine-grained crystals of cobalt-iron or nickel-iron alloy dispersedwithin a glassy matrix, the surface layer not exceeding several thousandangstroms in thickness and the crystals having diameters less than onemicron and, normally, less than 0.5 micron.

The glass-ceramic articles have an overall composition consistingessentially, in weight percent on the oxide basis, of about 2.5-5.5% Li₂O, 16-27% Al₂ O₃, 1-4% FeO, 0.5-3% NiO and/or 0.75-4% CoO, the sum ofCoO+NiO not exceeding about 5%, 40-75% SiO₂, and 1.75-6% TiO₂. Up toabout 3% ZrO₂ may optionally be included to assist nucleation.

Whereas minor amounts of glass-forming oxides such as B₂ O₃ and P₂ O₅,as well as such divalent metal oxides as the alkaline earth metaloxides, PbO, and ZnO, can be tolerated in small amounts to improve themelting and forming capabilities of the precursor glass or to modify thefinal physical properties, the most preferred compositions will consistessentially solely of the above-recited base compositions.

In general, the beta-spodumene solid solutions have compositions withinthe Li₂ O.Al₂ O₃.nSiO₂ system wherein n will vary between about 3.5-8.

The inventive method for making the composite glass articlescontemplates four basic elements:

(1) a glass-forming batch of the proper proportions is melted;

(2) the melt is simultaneously cooled below the transformation rangethereof (optionally to room temperature) and a glass article of adesired geometry shaped therefrom;

(3) the glass article is exposed in an oxidizing environment to atemperature between about 725°-875° C. for a sufficient length of timeto cause the growth in situ of CoFe₂ O₄, NiFe₂ O₄, or (Co,Ni)Fe₂ O₄crystallites in an integral surface layer on the article, therebyproducing a composite glass article; and then

(4) the composite glass article is subjected to a reducing environmentat a temperature between about 500°-800° C. for a sufficient length oftime to reduce the CoFe₂ O₄, NiFe₂ O₄, or (Co,Ni)Fe₂ O₄ crystallites tocrystals of cobalt-iron, nickel-iron, or cobalt-nickel-iron alloy.

The inventive method for preparing the composite glass-ceramic articlesalso comprehends four fundamental steps:

(1) a glass-forming batch of the proper proportions is melted;

(2) the melt is simultaneously cooled below the transformation rangethereof (optionally to room temperature) and a glass article of adesired geometry shaped therefrom;

(3) the glass article is exposed in an oxidizing environment to atemperature between about 900°-1300° C. for a sufficient length of timeto cause the growth in situ of CoFe₂ O₄, NiFe₂ O₄, or (Co,Ni)Fe₂ O₄crystallites in an integral surface layer on the article and the growthin situ of crystals of beta-spodumene solid solution in the body portionof the article, thereby producing a composite glass-ceramic article; andthen

(4) the composite glass-ceramic article is subjected to a reducingenvironment at a temperature between about 500°-800° C. for a sufficientlength of time to reduce the CoFe₂ O₄, NiFe₂ O₄, or (Co,Ni)Fe₂ O₄crystallites to crystals of cobalt-iron, nickel-iron, orcobalt-nickel-iron alloy.

The alloy layers are continuous over the surface of the glass andglass-ceramic articles and provide electrical conduction over a widerange of values depending upon precursor glass composition, heattreatment, and surface conditions; these values, however, being wellwithin the range commonly associated with metallic conductors. Also,because of this continuity of the surface layer, the products can beefficient solar selective absorbers for solar energy applications.

Because crystal growth in situ is well-known to contemplate atime-temperature relationship, extensive crystallization will demandlonger exposures to temperatures at the cooler extreme of thecrystallization range than at the upper end thereof. For example, in theformation of the CoFe₂ O₄, NiFe₂ O₄, or (Co,Ni)Fe₂ O₄ crystallites inthe surface of the glass articles, exposures of only about 1-2 hours maybe necessary at the higher end of the crystallization temperatureinterval, whereas up to 24 hours and longer may be required at the lowertemperatures of the effective range. Likewise, in the growth ofbeta-spodumene solid solution crystals concomitantly with thedevelopment of CoFe₂ O₄, NiFe₂ O₄, or (Co,Ni)Fe₂ O₄ crystals in thesurface layer of the glass-ceramic article, longer exposure periods willbe demanded at temperatures approaching 900° C. than at temperatures inthe higher extreme of the range. It is quite customary to develop thecrystallization present in the glass-ceramic articles via two generalsteps; viz., a nucleation heat treatment at temperatures somewhat abovethe transformation range, i.e., at temperatures about 750°-850° C. forabout 1-6 hours, followed by the crystallization treatment at 900°-1300°C. for about 3-12 hours.

In general, reduction of the CoFe₂ O₄, NiFe₂ O₄, or (Co,Ni)Fe₂ O₄crystallites to crystals of cobalt-iron, nickel-iron, orcobalt-nickel-iron alloy will be achieved after about 2-8 hours'exposure to a reducing environment at temperatures within the 500°-800°C. interval. Longer periods of exposure do not appear to adverselyaffect the electrical and magnetic properties of the final product, butneither do such seem to lend any significant advantage. Customarily,after the treatment in the reducing environment has been concluded, thecomposite glass-ceramic article will be cooled to room temperature whilein the presence of a reducing environment.

It has been learned that the electrical and magnetic propertiesexhibited by the final product can often be significantly enhanced whenthe surface layer of the afore-mentioned CoFe₂ O₄, NiFe₂ O₄, or(Co,Ni)Fe₂ O₄ crystallites is contacted with a hot mineral acid prior tosubjecting the composite glass or glass-ceramic articles to the reducingenvironment. Accordingly, such contact constitutes the most desiredpractice of the invention. Hydrochloric acid, sulfuric acid, phosphoricacid, and nitric acid have been found operable for the purpose withnitric acid being preferred since the other three acids demonstrate sometendency to attack the ferrite surface, so must be utilized withcaution.

DESCRIPTION OF PREFERRED EMBODIMENTS

Table I reports several approximate glass compositions, expressed interms of parts by weight on the oxide basis, illustrating the parametersof the instant invention where either CoFe₂ O₄ or NiFe₂ O₄ ferrite isproduced. Compositions leading to the formation of (Co,Ni)Fe₂ O₄ and theternary cobalt-nickel-iron alloys are recorded in Table III, infra, andwill be discussed hereinbelow. Because it is not known with whichcation(s) the fluoride is combined, it is simply stated in the form ofAlF₃, the batch material by which the fluoride was added. Moreover,inasmuch as the sum of the individual ingredients equals or closelyapproaches 100, for all practical purposes the compositions may beconsidered to have been reported in terms of weight percent. Finally,the actual batch components can comprise any material, either the oxideor other compound, which, when melted together with the otherconstituents, will be converted into the desired oxide in the properproportion.

The batch ingredients were compounded in an amount to yield about 1000grams. The batches were ballmilled to aid in securing a homogeneous meltand then placed into platinum crucibles. The crucibles were covered,moved to a furnace operating at 1550°-1650° C., and the batches meltedfor about 16 hours with stirring. The melts were thereafter poured intoslabs having dimensions about 10"×1"×0.25" and those slabs immediatelytransferred to an annealer operating at 400°-650° C. A brief dwellperiod at the higher annealing temperature was commonly utilized torelieve strain in the glass and avoid surface devitrification.

As₂ O₃ was added to the several base compositions to function in itscustomary capacity as a fining agent.

                  TABLE I                                                         ______________________________________                                        1        2      3      4    5    6    7    8    9                             ______________________________________                                        SiO.sub.2                                                                           64.3   51.3   50.6 51.3 50.9 68.7 62.9 59.5 67.6                        Al.sub.2 O.sub.3                                                                    21.1   30.0   27.7 30.0 26.4 21.9 20.7 26.1 20.2                        Li.sub.2 O                                                                          5.1    --     --   --   --   4.1  2.7  5.4  4.1                         Na.sub.2 O                                                                          --     15.3   15.1 14.1 13.8 --   --   --   --                          FeO   2.1    1.8    1.8  2.7  1.9  3.0  1.9  2.4  2.7                         CoO   1.1    0.9    0.9  1.4  --   --   --   --   --                          NiO   --     --     --   --   1.0  2.1  2.0  2.5  0.9                         TiO.sub.2                                                                           3.4    --     --   --   --   --   2.1  3.5  4.0                         As.sub.2 O.sub.3                                                                    0.6    0.8    0.7  0.4  0.7  0.3  0.3  0.7  0.5                         AlF.sub.3                                                                           2.4    --     3.1  --   2.9  --   --   --   --                          B.sub.2 O.sub.3                                                                     --     --     --   --   2.4  --   --   --   --                          P.sub.2 O.sub.5                                                                     --     --     --   --   --   --   7.4  --   --                          ______________________________________                                    

In these laboratory examples, the glass slabs were annealed to roomtemperature to enable inspection of glass quality and to allow sawing ofthe bodies into test samples. That practice is not necessary for thesuccessful operation of the subject invention, but the glass articlesmust be cooled to a temperature below the transformation range thereofbefore being heat treated in order to insure the development in situ offine-grained surface crystallization with or without fine-grained bodycrystallization. The transformation range has been defined as thattemperature at which a liquid melt is converted into an amorphous solid,that temperature being considered to lie in the vicinity of theannealing point of the glass.

Table II sets forth several heat treatment schedules in an airatmosphere to which glass articles prepared from the exemplarycompositions of Table I were exposed to convert them to composite glassand glass-ceramic articles having very thin, integral surface layerscontaining crystallites of CoFe₂ O₄ or NiFe₂ O₄. It will be appreciatedthat the rates at which the articles were heated to the citedtemperatures are illustrative only and must not be deemed limitative.The rates of heating are guided by the desire to avoid cracking orbreaking caused by thermal shock and, at temperatures approaching andexceeding the softening point of the glass where a glass-ceramic isbeing produced, to avoid deformation of the article due to incompletecrystal growth. Likewise, the heat treating temperature utilized is amatter of empirical choice and must not be considered limitative. Thedetermination of suitable heat treating schedules is well within thetechnical ingenuity of the worker of ordinary skill in the art.

Table II also reports several treatments in reducing environments whichwere applied to the composite glass and glass-ceramic articles totransform the CoFe₂ O₄ or NiFe₂ O₄ crystallites to cobalt-iron ornickel-iron alloy. Various gaseous atmospheres are suitable as reducingenvironments, the most useful being hydrogen and mixtures of hydrogenand nitrogen. In the examples recorded in Table II dry forming gas,i.e., 92% by volume nitrogen and 8% by volume hydrogen, constituted thereducing gaseous atmosphere. This mixture of gases is especiallypreferred because it does not present the safety hazards encounteredwith hydrogen alone and is relatively inexpensive. After baking atelevated temperatures and purging a heated furance tube with dry forminggas to remove any tramp gas therefrom, the specimens reported in TableII were exposed to a flow rate of the gas of about 100 cc/minute.

A treatment of at least about two hours at 500°-800° C. in the forminggas environment appears necessary to secure essentially completeconversion of the ferrite crystals to alloy. Exposures longer than abouteight hours can be utilized but with no apparent improvement in crystalmicrostructure or physical properties exhibited. Therefore, an exposureof eight hours is deemed to comprise a practical maximum period.

Whereas not mandatory, the electrical and magnetic properties of thealloy surfaces appear to be improved where the ferrite surface layersare contacted with a mineral acid such as nitric acid, hydrochloricacid, phosphoric acid, or sulfuric acid. This phenomenon is especiallyapparent in those Li₂ O-containing glass and glass-ceramic articles. Hotconcentrated and dilute nitric acid, dilute sulfuric acid, dilutephosphoric acid, and dilute hydrochloric acid can be operable. Hotconcentrated hydrochloric acid, phosphoric acid, and sulfuric acid tendto dissolve the ferrite layer and, hence, must be used with extremecaution, if employed at all. The reaction mechanism by means of whichthe acid treatment enhances the electrical and magnetic properties ofthe alloy layer has not been fully elucidated, but the treatment isthought to remove tramp components introduced through manual handling ofthe samples and/or exposure to the ambient atmosphere. Furthermore, thetreatment is believed to insure the removal of any glassy film, commonlyless than 100 A in thickness, at the surface. In the examples tabulatedbelow, boiling concentrated nitric acid was employed since it appearedto significantly improve the electrical and magnetic properties of thefinal products without seriously degrading other properties of thespecimens. Immersions in such acid baths much in excess of two minutesbegin to attack the ferrite surfaces, however. The use of dilute acidmay be desirable since it can be as effective as the concentrated acidand less care need be taken with its use, because it will not attack thesurface layer as rapidly.

The Na₂ O-containing glass articles are quite sensitive to attack byacids so contact of said glasses with acids must be very brief, if atall. For example, immersions of no more than about two minutes in aconcentrated acid bath will initiate visible corrosion of the glass.

Also reported in Table II are the crystal phases present in the interiorportion of the glass-ceramic articles as identified through X-raydiffraction analyses. Examination of the surface layers on the glass andglass-ceramic articles having NiO in the composition via X-ray andelectron diffraction analyses readily identified the presence ofnickel-iron alloy. However, there is no listing for a cobalt-iron alloyin the Index to the Powder Diffraction File, 1976 Edition, AmericanSociety for Testing Materials. Accordingly, the existence of this alloywas determined empirically in the following manner.

Surfaces from CoO-containing glasses and glass-ceramic articles made inaccordance with the inventive method were removed via treatment inboiling concentrated hydrochloric acid and the resultant solutionsevaporated to dryness. The presence of cobalt and iron in the residuewas confirmed spectrographically. To prove the existence of thecobalt-iron alloy, the material was synthesized by first forming theferrite and then reducing the ferrite to the alloy in forming gas. Thus,a stoichiometric mixture of Co₃ O₄ and Fe₂ O₃ was reacted in air at1400° C. for 20 hours. An X-ray diffraction pattern of the resultingproduct revealed only CoFe₂ O₄ with no unreacted oxides or otherimpurity phases present. The powdered ferrite was thereafter heated forthree hours in forming gas at 525° C. The product was a ferromagnetic,electrically-conducting material with the structure of alpha-iron. Thatthe material contained cobalt and iron atoms was verified analytically.The X-ray diffraction pattern showed only the CoFe₂ alloy phase; noferrite, metallic cobalt, metallic iron, or impurity oxide phase wasdetected. The diffraction pattern of the synthesized alloy matches thatobserved for the reduced Co-Fe surfaces on the inventive compositeglasses and glass-ceramics.

The remanent flux and coercive force exhibited by the inventive productswere measured in the following manner. The composite glass andglass-ceramic articles were magnetized and the magnetic propertiesdetermined by applying a strong magnetic field thereto. The strength ofthe applied field was increased until the test specimen was magneticallysaturated. Thereupon, the applied field was reduced to zero and thedegree of permanent magnetism of the sample determined in terms ofremanent magnetic flux. The coercive force required to demagnetize thespecimen was measured through the application of a magnetic field ofincreasing strength with reverse polarity. Remanent flux is measured inmaxwells/cm and coercive force is oersteds.

The optical properties reported in Table II are related to the solarspectrum. Thus:

α=integrated absorptance of solar radiation from 0.3-2.0 microns

ε=integrated emittance of thermal radiative heat loss from about 2.5-30microns

The electrical properties listed in Table II include:

σ=surface resistivity in ohms/square at 25° C. and 200° C.

TCR=temperature coefficient of resistivity in PPM/°C. for alloysurfaces=

    σ1/25-(.sup.σ 200-.sup.σ 25)/175×10.sup.6

The treatments recited in Table II were carried out on precursor glassbodies which had been annealed to room temperature to permit examinationfor glass quality and to allow test samples to be sawed therefrom. Itwill be appreciated that this cooling to room temperature is not amandatory step of the invention. It is necessary, however, for theoriginal glass article to be cooled to a temperature below thetransformation range of the glass prior to the crystallization heattreatment in order to insure the development of uniformly veryfine-grained crystals, both in the interior of the glass-ceramicarticles and in the integral surface layers.

Likewise, the use of specific dwell periods at particular temperaturesis not demanded. It is only necessary that the articles be exposed totemperatures within the cited effective ranges for a period of timesufficient to generate the desired crystallization.

                                      TABLE II                                    __________________________________________________________________________    Example                                                                            Heat Treatment                                                                            Acid Treatment                                                                          Reducing Heat Treatment                                                                    Article Interior                      __________________________________________________________________________    1    300° C./hour to 900° C.                                                     --        --           Beta-spodumene solid solution              Hold 4 hours                                                             1    300° C./hour to 900° C.                                                     None      600° C./hour to 600° C.                                                      Beta-spodumene solid solution              Hold 4 hours          Hold 2 hours                                       2    300° C./hour to 750° C.                                                     0.5 minute in boiling                                                                   300° C./hour to 525° C.                                                      Glass                                      Hold 12 hours                                                                             concentrated HNO.sub.3                                                                  Hold 3 hours                                       3    300° C./hour to 800° C.                                                     None      300° C./hour to 550° C.                                                      Glass                                      Hold 16 hours         Hold 2 hours                                       4    300° C./hour to 750° C.                                                     0.5 minute in boiling                                                                   300° C./hour to 525° C.                                                      Glass                                      Hold 12 hours                                                                             concentrated HNO.sub.3                                                                  Hold 3 hours                                       5    500° C./hour to 800° C.                                                     None      600° C./hour to 525° C.                                                      Glass                                      Hold 16 hours         Hold 2 hours                                       6    300° C./hour to 825° C.                                                     --        --           Glass                                 6    300° C./hour to 825° C.                                                     None      600° C./hour to 700° C.                                                      Glass                                      Hold 16 hours         Hold 4 hours                                       7    300° C./hour to 800° C.                                                     None      300° C./hour to 525° C.                                                      Glass                                      Hold 6 hours          Hold 5 hours                                       8    300° C./hour to 700° C.                                                     0.5 minute in boiling                                                                   300° C./hour to 525° C.                                                      Beta-spodumene solid solution              20° C./hour to 800° C.                                                      concentrated HNO.sub.3                                                                  Hold 3 hours                                            Hold 4 hours                                                             __________________________________________________________________________    Example                                                                            Article Surface                                                                       Remanent Flux                                                                         Coercive Force                                                                        °25                                                                          °200                                                                          TCR                                                                              α                                                                              ε                 __________________________________________________________________________    1    CoFe.sub.2 O.sub.4                                                                    0.065   689     --    --     -- 0.9    0.8                       1    Co-Fe alloy                                                                           0.289   580     --    --     -- 0.94   0.36                      2    Co-Fe alloy                                                                           0.053   909     --    --     -- --     --                        3    Co-Fe alloy                                                                           --      --      --    --     -- 0.9    0.46                      4    Co-Fe alloy                                                                           0.081   768     --    --     -- --     --                        5    Ni-Fe alloy                                                                           --      --      --    --     -- 0.88   0.57                      6    NiFe.sub. 2 O.sub.4                                                                   --      --      1.8 ×10.sup.10                                                                2.5 ×10.sup.6                                                                  -- --     --                        6    Ni-Fe alloy                                                                           0.106   306     0.7   1.8    9000                                                                             0.90   0.66                      7    Ni-Fe alloy                                                                           --      --      1.0   1.6    3400                                                                             --     --                        8    Ni-Fe alloy                                                                           0.676   230     12.8  --     -- --     --                        __________________________________________________________________________    Example                                                                            Heat Treatment                                                                            Acid Treatment                                                                          Reducing Heat Treatment                                                                    Article Interior                      __________________________________________________________________________    8    300° C./hour to 750° C.                                                     --        --           Beta-spodumene solid solution              20° C./hour to 850° C.                                          100° C./hour to 1250° C.                                        Hold 12 hours                                                            8    300° C./hour to 750° C.                                                     None      300° C./hour to 800° C.                                                      Beta-spodumene solid solution              20° C./hour to 850° C.                                                                Hold 5 hours                                            1000° C./hour to 1250° C.                                       Hold 12 hours                                                            9    300° C./hour to 750° C.                                                     0.5 minute in boiling                                                                   600° C./hour to 600° C.                                                      Beta-spodumene solid solution              25° C./hour to 1250° C.                                                     concentrated HNO.sub.3                                                                  Hold 5 hours                                            Hold 6 hours                                                             9    300° C./hour to 750° C.                                                     None      300° C./hour to 650° C.                                                      Beta-spodumene solid solution              25° C./hour to 850° C.                                                                Hold 2 hours                                            100° C./hour to 1225° C.                                        Hold 8 hours                                                             __________________________________________________________________________    Example                                                                            Article Surface                                                                       Remanent Flux                                                                         Coercive Force                                                                        °25                                                                          °200                                                                          TCR                                                                              α                                                                              ε                 __________________________________________________________________________    8    NiFe.sub.2 O.sub.4                                                                    --      --      --    --     -- 0.80   0.82                      8    Ni-Fe alloy                                                                           --      --      1.9   2.5    4800                                                                             0.85   0.67                      9    Ni-Fe alloy                                                                           0.269   321     --    --     -- --     --                        9    Ni-Fe alloy                                                                           --      --      7.3   9.8    1950                                                                             --     --                        __________________________________________________________________________

Several pertinent observations can be drawn from an inspection of TableII. Thus, Example 1 illustrates that the remanent magnetization of theferromagnetic alloy is greater than that of the ferrimagnetic ferrite.The other examples demonstrate the wide range of magnetic propertiesthat can be developed as a function of composition and heat treatment.Also, the coercive force of the Ni-Fe alloy appears to be considerablyless than that of the Co-Fe alloy.

An efficient solar selective absorber for solar energy applications willhave a high α and low ε. For temperatures below 500° C., 98% of thethermal radiation occurs at wave lengths greater than two microns.Examples 1 and 8 manifest that the emittance of the alloy surface isless than that of the precursor ferrite surface. A comparison of thealloy surface emittances of Examples 1 and 3 with those of Examples 5,6, and 8 indicates the Co-Fe alloy surfaces to have somewhat loweremittance than Ni-Fe alloy surfaces. Low emittance is conventionallyassociated with low electrical resistivity, i.e., high electricalconductivity, and is confined to semiconductors and metals.

The surface resistivity of the ferrite surfaces at room temperature isin excess of 2×10⁷ ohms/square. The nickel ferrite surface of Example 6has a surface resistivity of about 10¹⁰ ohms/square at 25° C. comparedto about one ohm/square for the Ni-Fe alloy. The great decrease inelectrical resistivity with temperature exhibited by the nickel ferritesurface of Example 6 contrasts sharply with the increase in electricalresistivity with temperature demonstrated by the alloy surfaces inExamples 6-9.

In an attempt to better understand the mechanism of formation of theferrimagnetic compound ferrites of the type (Co,Ni)Fe₂ O₄ and theirsubsequent reduction to ternary alloys, the following laboratoryinvestigation was undertaken. Table III records several approximateglass compositions in like manner to Table I above, expressed in termsof parts by weight on the oxide basis. Again, the fluoride content issimply reported as AlF₃, the actual batch ingredient, and, because thetotal of the listed materials approaches 100, the compositions can bedeemed to be in terms of weight percent for all practical purposes.

The batch ingredients were compounded, melted, and the melts shaped andannealed in accordance with the procedure outlined above with respect tothe exemplary compositions of Table I.

                  TABLE III                                                       ______________________________________                                        10          11     12        13   14      15                                  ______________________________________                                        SiO.sub.2                                                                           63.0      63.0   60.3    60.3 60.5    60.5                              B.sub.2 O.sub.3                                                                     3.0       3.0    2.8     2.8  2.8     2.8                               Al.sub.2 O.sub.3                                                                    21.7      21.7   20.7    20.7 20.8    20.8                              AlF.sub.3                                                                           2.4       2.4    2.3     2.3  2.3     2.3                               Li.sub.2 O                                                                          4.2       4.2    4.1     4.1  4.1     4.1                               FeO   3.3       2.0    3.1     2.0  3.1     2.0                               CoO   1.0       1.6    0.9     1.5  0.9     1.5                               NiO   1.0       1.6    0.9     1.5  0.9     1.5                               TiO.sub.2                                                                           --        --     4.3     4.3  2.0     2.0                               ZrO.sub.2                                                                           --        --     --      --   2.0     2.0                               As.sub.2 O.sub.3                                                                    0.6       0.6    0.6     0.6  0.6     0.6                               ______________________________________                                    

Examples 10 and 11 are stable glasses, whereas Examples 12 and 13 areTiO₂ nucleated and Examples 14 and 15 are TiO₂ -ZrO₂ nucleated,thermally devitrifiable glasses, i.e., Examples 12-15 can be convertedinto glass-ceramic articles.

Table IV recites the heat treatment schedules in an air atmosphereapplied to the glass articles of Table III to transform them intocomposite glass articles (Examples 10-11) and glass-ceramic articles(Examples 12-15) having very thin, integral surface layers containingcrystallites of the type (Co,Ni)Fe₂ O₄. A description of the visualappearance of each specimen and the crystal phases identified in thesurface layer of each via X-ray diffraction analysis are also recorded.

Table IV further reports a firing of the composite articles in dryforming gas with no preliminary acid etch. A visual description of eachsample, the crystal phases identified in the surface layer of eachthrough X-ray diffraction analyses, and a measure of surfaceresistivity, σ, in ohms/square at 25° C. are also tabulated.

Finally, Table IV records a firing of the composite glass articles indry forming gas preceded by an acid etch in 10% aqueous HNO₃ andillustrates the sharp reduction in surface resistivity resulting fromthe acid etch.

The presence of all three transition metal ions, i.e., iron, cobalt, andnickel, in the surface layer of each example was demonstrated by (a)firing each sample in air, (b) reducing each fired sample in forminggas, and (c) stripping the metallic surface so formed via etching inHCl. In this latter step the specimens were immersed into boilingaqueous 50% HCl for a period of time until the evolution of hydrogen gasceased, normally a matter of about 10-20 seconds. Hydrogen is liberatedaccording to the reaction

    RO+2HCl.sub.aq →R.sub.aq.sup.+2 +2Cl.sub.aq.sup.- +H.sub.2 ↑

wherein R refers to Co⁺², Fe⁺², and Ni⁺². Cessation of hydrogenevolution indicates that the metallic phase has been completely removedfrom the surface layer. The specimens were thereafter lifted out of theetch bath to avoid attack of the glass or glass-ceramic substrate.

The etch bath was then evaporated to dryness and the residue subjectedto emission spectrographic analysis. Metallic cobalt, iron, and nickelwere determined to be present in each of the six samples. This resultwas confirmed via electron spectrometric analyses which also indicatedthe cobalt, iron, and nickel oxides or metallic species were presentalmost exclusively on the air-fired and reduced surfaces, respectively.

The air-fired surfaces of the stable glasses, viz., Examples 10-11,showed a weak ferrite crystal surface phase with Example 10 manifestingan additional trace amount of hematite. After firing in forming gas,Example 10 exhibited a weak diffraction pattern of an alloy having aγ-iron structure and elemental α-iron resulting from the reduction ofhematite. After reduction, Example 11 demonstrated only a weakdiffraction pattern of an alloy having a γ-iron structure.

The air-fired glass-ceramic specimens, viz., Examples 12-15, produced astrong diffraction pattern of ferrite crystals in the surface withbeta-spodumene solid solution constituting the predominant crystal phasein the interior. Minor to trace amounts of rutile, ZrTiO₄, hematite, anda spinel phase were sometimes observed in the substrate. Firing thesamples in forming gas reduced the surface ferrite crystals to an alloyexhibiting α-iron structure. A weak line in the diffraction pattern wasobserved at about 2.06 A, thereby suggesting a trace amount ofcrystallinity having a γ-iron structure. However, the absence of otherdiffraction lines casts doubt on this assignment.

                                      TABLE IV                                    __________________________________________________________________________    Example                                                                            Heat Treatment                                                                            Visual Description                                                                      Surface Crystallization                                                                   Interior Crystallization               __________________________________________________________________________    10   300° C./hour to 775° C.                                                     Black brown                                                                             Ferrite, hematite                                                                         None                                        Hold 5 hours                                                             10   300° C./hour to 775° C.                                                     --        --          --                                          Hold 5 hours                                                             10   300° C./hour to 775° C.                                                     --        --          --                                          Hold 5 hours                                                             11   300° C./hour to 775° C.                                                     Metallic gray                                                                           Ferrite     None                                        Hold 5 hours                                                             11   300° C./hour to 775° C.                                                     --        --          --                                          Hold 5 hours                                                             11   300° C./hour to 775° C.                                                     --        --          --                                          Hold 5 hours                                                             12   500° C./hour to 900° C.                                                     Charcoal  Ferrite     Beta-spodumene solid solution               Hold 5 hours                                                             12   500° C./hour to 900° C.                                                     --        --          --                                          Hold 16 hours                                                            12   500° C./hour to 900° C.                                                     --        --          --                                          Hold 16 hours                                                            Example                                                                            Acid Heat Treatment                                                                      Reducing Heat Treatment                                                                    Visual Description                                                                      Surface Crystallization                                                                   σ                    __________________________________________________________________________    10   --         --           --        --          --                         10   None       600° C./hour to 550° C.                         iron 1270                    Charcoal  γ-alloy + α                                Hold 5 hours                                                  10   1 minute in boiling                                                                      600° C./hour to 550° C.                         iron 79                      Charcoal  γ-alloy + α                     10% HNO.sub.3                                                                            Hold 5 hours                                                  11   --         --           --        --          --                         11   None       600° C./hour to 550° C.                                                      Gray      γ-alloy                                                                             2063                                       Hold 5 hours                                                  11   1 minute in boiling                                                                      600° C./hour to 550° C.                                                      Gray      γ-alloy                                                                              230                            10% HNO.sub.3                                                                            Hold 5 hours                                                  12   --         --           --        --          --                         12   None       600° C./hour to 550° C.                                                      Charcoal  α-alloy                                                                              12                                        Hold 5 hours                                                  Example                                                                            Heat Treatment                                                                            Visual Description                                                                      Surface Crystallization                                                                   Interior Cyrstallization               __________________________________________________________________________    13   500° C./hour to 900° C.                                                     Charcoal  Ferrite     Beta-spodumene solid solution               Hold 16 hours                                                            13   500° C./hour to 900° C.                                                     --        --          --                                          Hold 16 hours                                                            13   500° C./hour to 900° C.                                                     --        --          --                                          Hold 16 hours                                                            14   500° C./hour to 900° C.                                                     Charcoal  Ferrite     Beta-spodumene solid solution               Hold 16 hours                                                            14   500° C./hour to 900° C.                                                     --        --          --                                          Hold 16 hours                                                            14   500° C./hour to 900° C.                                                     --        --          --                                          Hold 16 hours                                                            15   500° C./hour to 900° C.                                                     Charcoal  Ferrite     Beta-spodumene solid solution               Hold 16 hours                                                            15   500° C./hour to 900° C.                                                     --        --          --                                          Hold 16 hours                                                            15   500° C./hour to 900° C.                                                     --        --          --                                          Hold 16 hours                                                            Example                                                                            Acid Heat Treatment                                                                      Reducing Heat Treatment                                                                    Visual Description                                                                      Surface Crystallization                                                                   σ                    __________________________________________________________________________    13   --         --           --        --          --                         13   None       600° C./hour to 550° C.                                                      Charcoal  α-alloy                                                                             15                                         Hold 5 hours                                                  14   --         --           --        --          --                         14   None       600° C./hour to 550° C.                                                      Charcoal  α-alloy                                                                             15                                         Hold 5 hours                                                  15   --         --           --        --          --                         15   None       600° C./hour to 550° C.                                                      Charcoal  α-alloy                                                                             21                                         Hold 5 hours                                                  __________________________________________________________________________

To further indicate the surface character of the inventive products,additional glass specimens of Examples 10, 11, 12, and 15 were fired inair at 500° C./hour to 900° C. and held at that temperature for 16hours. Each sample displayed the same visual appearance as set forth inTable IV above the demonstrated typical hysteresis loops, therebyindicating good ferrimagnetic properties resulting from the ferritesurface crystallization. The samples of Examples 10 and 11 were thenimmersed for one minute into boiling 10% HNO₃ and subsequently heated indry forming gas at 600° C./hour to 550° C. and maintained therewithinfor five hours. The specimens of Examples 12 and 15 were not subjectedto a preliminary acid immersion but were immediately exposed to the dryforming gas treatment schedule. All four bodies exhibited typicalhysteresis loops, thereby demonstrating ferromagnetic behavior as aresult of the presence of alloy crystals in the surface.

The ferrite and alloy crystal structure assignments recorded above inTable IV were determined via the synthesis of appropriate materials.Thus, it has been shown that the reduction of cobalt ferrite yields aCoFe₂ alloy having a structure similar to that of α-iron. Also, it hasbeen shown that the reduction of nickel ferrite results in a NiFe₂ alloyhaving a structure similar to that of γ-iron.

A ferrite having the stoichiometry CoO.NiO.2Fe₂ O₃, i.e., CoNiFe₄ O₈,was synthesized using (a) raw batch and (b) an equimolar mixture ofpreviously prepared CoFe₂ O₄ and NiFe₂ O₄. After firing each mixture at1400° C. for 16 hours, both gave an identical, simple ferritediffraction pattern which could readily be distinguished from thediffraction pattern derived from an unsintered mixture of the twoferrites. Upon reduction in dry forming gas for five hours at 700° C.,the compound ferrite was converted to the γ-iron alloy structure. Uponadditional firing in forming gas for five hours at 750° C., the crystalstructure was transformed into the α-iron form. This phenomenonindicates that the same ternary Co-Ni-Fe alloy can exist in both forms,which behavior is consistent with the findings for the alloy surfaces onglasses (γ-form) and glass-ceramics (α-form).

In summary, diffusion of iron, nickel, and cobalt to the surface canoccur in glasses and glass-ceramics to produce a single compound ferritesurface phase. Upon reduction, this phase converts to a alloy having theγ-iron structure on low temperature, stable glasses and to an alloyhaving the α-iron structure on glass-ceramic surfaces. Table IV alsopoints out that the surface resistivity of the glass articles ismarkedly lowered through the application of an acid etch prior to thereduction treatment.

I claim:
 1. A composite glass article composed of a glassy body portionand a highly crystalline, integral surface layer not exceeding severalthousand angstroms in thickness demonstrating ferromagnetic properties,said surface layer consisting essentially of cobalt-iron, nickel-iron,or cobalt-iron-nickel alloy crystals having diameters less than onemicron dispersed within a glassy matrix, said composite glass articlehaving an overall composition consisting essentially, in weight percenton the oxide basis, of about 1-17% R₂ O, wherein R₂ O consists of Li₂ Oand/or Na₂ O in the following indicated proportions of 2.5-5.5% Li₂ Oand 1-16% Na₂ O, 1-3.75% FeO, 0.75-5% RO, wherein RO consists of NiOand/or CoO in the following indicated proportions of 0.75-3% NiO and0.75-4% CoO, 20-32% Al₂ O₃, and 50-72% SiO₂.
 2. A composite glassarticle according to claim 1 wherein said overall composition alsocontains up to 10% P₂ O₅, and/or up to 3% B₂ O₃, and/or up to 2% F.
 3. Acomposite glass-ceramic article composed of an interior portion and anintegral surface layer not exceeding several thousand angstroms inthickness demonstrating ferromagnetic properties, said interior portionbeing highly crystalline and consisting essentially of beta-spodumenesolid solution crystals having compositions within the Li₂ O.Al₂ O₃.nSiO₂ system, wherein n will vary between about 3.5-8, dispersed within aglassy matrix, and said surface layer being highly crystalline andconsisting essentially of cobalt-iron, nickel-iron, orcobalt-nickel-iron alloy crystals having diameters less than one microndispersed within a glassy matrix, said glass-ceramic article having anoverall composition in weight percent on the oxide basis of about2.5-5.5% Li₂ O, 16-27% Al₂ O₃, 1-4% FeO, 0.5-3% NiO and/or 0.75-4% CoO,the sum of NiO+CoO not exceeding about 5%, 40-76% SiO₂, and 1.75-6% TiO₂in those articles wherein beta-spodumene solid solution is thepredominant crystal phase.